GPS Global Positiong System Modernization

GPS modernization

The United States' Global Positioning System (GPS), having reached Fully Operational Capability on July 17, 1995^[1] has completed its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to modernize the GPS system. Announcements from the Vice President and the White House in 1998 initiated these changes. In 2000, U.S. Congress authorized the effort, referred to as GPS III.
The project involves new ground stations and new satellites, with additional navigation signals for both civilian and military users, and aims to improve the accuracy and availability for all users.
Lockheed Martin was awarded the GPS III Space Segment contract on May 15, 2008. The first launch is projected for 2014.^[2] Raytheon was awarded the Next Generation GPS Control Segment (OCX) contract on Feb 25, 2010.^[3]

New Navigation Signals

Civilian L2 (L2C)

One of the first announcements was the addition of a new civilian-use signal to be transmitted on a frequency other than the L1 frequency used for the existing GPS Coarse Acquisition (C/A) signal. Ultimately, this became known as the L2C signal because it is broadcast on the L2 frequency (1227.6 MHz). It is transmitted by all block IIR-M and later design satellites.
The L2C signal is tasked with providing improved accuracy of navigation, providing an easy-to-track signal, and acting as a redundant signal in case of localized interference.
The immediate effect of having two civilian frequencies being transmitted from one satellite is the ability to directly measure, and therefore remove, the ionospheric delay error for that satellite. Without such a measurement, a GPS receiver must use a generic model or receive ionospheric corrections from another source (such as a Satellite Based Augmentation System). Advances in technology for both the GPS satellites and the GPS receivers have made ionospheric delay the largest source of error in the C/A signal. A receiver capable of performing this measurement is referred to as a dual frequency receiver. The technical characteristics of it are:

• L2C contains two distinct PRN sequences:
  • CM (for Civilian Moderate length code) is 10,230 bits in length, repeating every 20 milliseconds.
  • CL (for Civilian Long length code) is 767,250 bits, repeating every 1500 milliseconds (i.e., every 1.5 s).
  • Each signal is transmitted at 511,500 bits per second (bit/s), however they are multiplexed to form a 1,023,000 bit/s signal.
• CM is modulated with a 25 bit/s navigation message with forward error correction, whereas CL is a non-data sequence (it does not contain additional modulated data).
• The long, non-data CL sequence provides for approximately 24 dB greater correlation protection (~250 times stronger) than L1 C/A.
• L2C signal characteristics provide 2.7 dB greater data recovery and 0.7 dB greater carrier tracking than L1 C/A
• The L2C signals' transmission power is 2.3 dB weaker than the L1 C/A signal.
• In a single frequency application, L2C has 65% more ionospheric error than L1.

It is defined in IS-GPS-200.^[4]

Military (M-code)

A major component of the modernization process, a new military signal called M-code was designed to further improve the anti-jamming and secure access of the military GPS signals. The M-code is transmitted in the same L1 and L2 frequencies already in use by the previous military code, the P(Y) code. The new signal is shaped to place most of its energy at the edges (away from the existing P(Y) and C/A carriers).
Unlike the P(Y) code, the M-code is designed to be autonomous, meaning that users can calculate their positions using only the M-code signal. P(Y) code receivers must typically first lock onto the C/A code and then transfer to lock onto the P(y)-code.
In a major departure from previous GPS designs, the M-code is intended to be broadcast from a high-gain directional antenna, in addition to a wide angle (full Earth) antenna. The directional antenna's signal, termed a spot beam, is intended to be aimed at a specific region (i.e. several hundred kilometers in diameter) and increase the local signal strength by 20 dB (10X voltage field strength, 100X power). A side effect of having two antennas is that the GPS satellite will appear to be two GPS satellites occupying the same position to those inside the spot beam.
While the full-Earth M-code signal is available on the Block IIR-M satellites, the spot beam antennas will not be available until the Block III satellites are deployed, tentatively in 2013.
Other M-code characteristics are:

• Satellites will transmit two distinct signals from two antennas: one for whole Earth coverage, one in a spot beam.
• Modulation is binary offset carrier
• Occupies 24 MHz of bandwidth
• It uses a new MNAV navigational message, which is packetized instead of framed, allowing for flexible data payloads
• There are four effective data channels; different data can be sent on each frequency and on each antenna.
• It can include FEC and error detection
• The spot beam is ~20 dB more powerful than the whole Earth coverage beam
• M-code signal at Earth's surface: –158 dBW for whole Earth antenna, –138 dBW for spot beam antennas.

Safety of Life (L5)

Safety of Life is a civilian-use signal, broadcast on the L5 frequency (1176.45 MHz). In 2009, a WAAS satellite sent the initial L5 signal test transmissions. SVN-62, the first GPS block IIF satellite, continuously broadcast the L5 signal starting on June 28, 2010.

• Improves signal structure for enhanced performance
• Higher transmission power than L1 or L2C signal (~3dB, or twice as powerful)
• Wider bandwidth, yielding a 10-times processing gain
• Longer spreading codes (10 times longer than used on the C/A code)
• Located in the Aeronautical Radionavigation Services band, a frequency band that is available world wide.

WRC-2000 added space signal component to this aeronautical band so aviation community can manage interference to L5 more effectively than L2
It is defined in IS-GPS-705.^[5]

New Civilian L1 (L1C)

L1C is a civilian-use signal, to be broadcast on the same L1 frequency (1575.42 MHz) that currently contains the C/A signal used by all current GPS users. The L1C will be available with first Block III launch, currently scheduled for 2013.

• Implementation will provide C/A code to ensure backward compatibility
• Assured of 1.5 dB increase in minimum C/A code power to mitigate any noise floor increase
• Non-data signal component contains a pilot carrier to improve tracking
• Enables greater civil interoperability with Galileo L1

It is defined in IS-GPS-800.^[6]

Block III satellite improvements

Increased signal power at the Earth's surface

• M-code: –158 dBW / –138 dBW.
• L1 and L2: –157 dBW for the C/A code signal and –160 dBW for the P(Y) code signal.
• L5 will be –154 dBW.

Researchers from The Aerospace Corporation confirmed that the most efficient means to generate the high-power M-code signal would entail a departure from full-Earth coverage, characteristic of all the user downlink signals up until that point. Instead, a high-gain antenna would be used to produce a directional spot beam several hundred kilometers in diameter. Originally, this proposal was considered as a retrofit to the planned Block IIF satellites. Upon closer inspection, program managers realized that the addition of a large deployable antenna, combined with the changes that would be needed in the operational control segment, presented too great a challenge for the existing system design^[7]

• NASA has requested that Block III satellites carry laser retro-reflectors.^[8] This allows tracking the orbits of the satellites independent of the radio signals, which allows satellite clock errors to be disentangled from ephemeris errors. This is a standard feature of GLONASS, will be included in the Galileo positioning system, and was included as an experiment on two older GPS satellites (satellites 35 and 36).^[9]

• The USAF is working with NASA to add a DASS payload to the second increment of GPS III satellites as part of the MEOSAR system.^[10]